Electro-and electroless plating of metal in the manufacture of PCRAM Devices

ABSTRACT

Non-volatile, resistance variable memory devices, integrated circuit elements, and methods of forming such devices are provided. According to one embodiment of a method of the invention, a memory device can be fabricated by depositing a chalcogenide material onto a first (lower) electrode, sputter depositing a thin diffusion layer of a conductive material over the chalcogenide material, diffusing metal from the diffusion layer into the chalcogenide material resulting in a metal-comprising resistance variable material, and then plating a conductive material to a desired thickness to form a second (upper) electrode. In another embodiment, the surface of the chalcogenide layer can be treated with an activating agent such as palladium, a conductive metal can be electrolessly plated onto the activated areas to form a thin diffusion layer, metal ions from the diffusion layer can be diffused into the chalogenide material to form a resistance variable material, and a conductive material plated over the resistance variable material to form the upper electrode. The invention provides a process for controlling the diffusion of metal into the chalcogenide material to form a resistance variable material by depositing the mass of the upper electrode by a metal plating technique.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. Ser. No. 09/956,783, filed Sep.20, 2001.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor fabricationtechniques and, more particularly, to a method for fabricatingelectrodes for use in phase changeable or resistance variable memorydevices such as, for example, chalcogenide-based memory cells.

BACKGROUND OF THE INVENTION

Memory devices are used in integrated circuitry to store information inthe form of binary data. There are various types of memory devicesincluding volatile semiconductor memory in which stored data is retainedas long as power to the system is not turned off, such as dynamic randomaccess memories (DRAMs). Non-volatile memories, such as read-onlymemories (ROMs), retain stored data even when power is discontinued, buthave less storage capability and programming options than volatilememories. Although there are non-volatile memories such as programmableread-only memory (PROMs) and electrically-erasable PROM (EEPROMs) thatpermit limited reprogramming, there are limits on the programmingcapacity of such memory devices.

Programmable metallization cells (PMCs) comprise a fast ion conductor orresistant variable material, typically a chalcogenide material havingmetal ions therein, which is disposed between two electrodes comprisingan electrically conducting material (e.g., an anode and a cathode), asdescribed, for example, in U.S. Pat. No. 6,084,796 (Kozicki et al., AxonTechnologies). Resistant variable materials (or fast ion conductors) arecapable of assuming a high resistance “off” and a low resistance “on”state in response to a stimulus for a binary memory, and multiplegenerally stable states in response to a stimulus for a higher ordermemory. The resulting memory element is non-volatile in that it willmaintain the integrity of the information stored by the memory cellwithout the need for periodic refresh signals, and the data integrity ofthe information stored by these memory cells is not lost when power isremoved from the device.

The resistant variable material (e.g., chalcogenide-metal ion material)undergoes a chemical and structural change at a certain applied voltage.Specifically, at a threshold voltage, plating of metal from metal ionsoccurs. A metal dendrite grows within the chalcogenide-metal ionmaterial, eventually connecting the two electrodes. The growth rate ofthe dendrite is a function of the applied voltage and time. The growthof the dendrite can be stopped by removing the voltage or the dendritecan be retracted back towards the cathode by reversing the voltagepolarity at the anode and cathode.

Changes in the length of the dendrite affect the resistance andcapacitance of the PMC. If dendrite growth is continued until iteffectively interconnects the electrodes to electrically short themtogether, a drop in the resistance of the resistance variable materialresults. The resistance variable material can be returned to a highlyresistive state by reversing the voltage potential between the anode andcathode, whereupon the dendrite is disrupted. Thus, such a device canfunction as a programmable memory cell of a memory circuitry.

An exemplary resistance variable material comprises germanium selenidewith silver ions diffused therein. Current methods provide silver ionswithin the germanium selenide material by initially depositing thegermanium selenide glass layer onto a substrate, typically a firstelectrode, and then depositing a thin overlying layer of silver,typically by physical vapor deposition (i.e., sputtering). The thinsilver layer is then exposed to electromagnetic energy such asultraviolet (UV) radiation to diffuse silver into the germanium selenidelayer, such that a homogenous distribution of silver throughout thelayer is ultimately achieved. In an exemplary embodiment, the upperelectrode is then formed from silver that is sputter deposited onto themetal-doped germanium selenide layer.

In the process of depositing silver to form the upper electrode, plasmagenerated during sputtering results in the generation of electromagneticradiation, which drives additional silver into the metal-doped material.Although some doping of silver into the material is needed to provide aworking device whereby silver from silver ions within the materialplates out to grow the dendrite extension between the two electrodes,the amount of electromagnetic radiation generated during the sputteringprocess can drive excessive amounts of silver into the resistancevariable material such that the device is rendered non-functional.

Therefore, a need exists for a process for fabricating memory cellscomprising a resistance variable material that avoids such problems.

SUMMARY OF THE INVENTION

The present invention relates generally to semiconductor fabricationtechniques and, more particularly, to non-volatile, resistance variablememory devices such as, for example, chalcogenide memory cells, andmethods of forming such devices, and more particularly to thefabrication of electrodes on resistance variable materials of anintegrated circuit element.

In one aspect, the invention provides methods of forming a memory devicecomprising a resistance variable material interposed between an upperand a lower electrode. In an embodiment of a method according to theinvention, a lower electrode layer is formed on a substrate, achalcogenide material is deposited onto the lower electrode, and a thindiffusion layer comprising a conductive metal, such as silver, is formedover the chalcogenide material for example, by physical vapor deposition(i.e., sputtering) or by chemical vapor deposition (CVD), and then aconductive metal is plated onto the diffusion layer to a desiredthickness to form the upper electrode. Metal ions from the diffusionlayer are diffused into the chalcogenide material prior to plating theupper electrode, resulting in a metal-comprising resistance variablematerial. In another embodiment, a thin seed layer of conductive metalis deposited onto the resistance variable material layer and/or thediffusion layer, followed by plating of a conductive metal to form theupper electrode.

In an exemplary embodiment of the method, a layer of germanium selenidematerial is deposited, for example, by chemical vapor deposition orsputtering, onto a lower electrode layer that can comprise, for example,silver, tungsten, platinum, or other conductive material. A silvermaterial is sputter deposited onto the germanium selenide layer to forma thin diffusion layer, preferably to a thickness of at least about 100angstroms to about 300 angstroms or less. The diffusion layer can thenbe exposed to electromagnetic radiation or other diffusion source todiffuse silver ions into the germanium selenide layer to form theresistance variable material. A conformal layer of a conductive materialsuch as silver is then plated over the diffusion layer by anelectroplating or an electroless plating process to a desired thicknessto form the upper electrode, typically about 500 to about 2000angstroms. A seed layer can be deposited as a base for the upperelectrode deposition, if appropriate.

In another embodiment of a method of the invention, the upper electrodeis formed over a chalcogenide material by first contacting thechalcogenide material with a surface activating agent to form anactivated surface area, and then electroless plating a conductive metal(e.g., silver) onto the activated surface area to form a thin diffusionlayer. An exemplary surface activating agent is palladium. Metal ionsfrom the plated diffusion layer can then be diffused into thechalcogenide material, resulting in a metal-comprising resistancevariable material. The upper electrode is then formed by electrolessplating or electroplating a conductive material (e.g., silver) to adesired thickness over the diffusion layer. A thin seed layer, ifappropriate, can be deposited by plating prior to plating of the upperelectrode.

In another aspect, the invention provides methods of diffusing metalions into a chalcogenide material to form a resistance variablematerial. One embodiment of the method comprises sputter or CVDdepositing a conductive metal material onto a chalcogenide material toform a thin diffusion layer, and treating the diffusion layer to diffusemetal ions from the conductive metal material into the chalcogenidematerial, for example, by exposure to electromagnetic radiation. Inanother embodiment of a method of diffusing metal into a chalcogenidematerial to form a resistance variable material, the chalcogenidematerial is contacted with a surface activating agent to form anactivated surface area thereon, a layer of metal is plated onto theactivated surface area by an electroless plating process to form adiffusion layer of about 100 to about 300 angstroms, and metal ions fromthe diffusion layer can then be diffused into the chalcogenide materialsuch as by exposure to electromagnetic radiation. In either embodiment,a layer of a conductive metal material can then be plated onto thediffusion layer to a desired thickness to form the upper electrode byelectroplating or electroless plating techniques. In another embodiment,a thin seed layer can be deposited as a base layer prior to plating themetal electrode layer. In an exemplary embodiment, the chalcogenidematerial comprises germanium selenide, and both of the diffusion layerand the electrode layer comprise silver. The diffusion layer ispreferably deposited to a thickness of about 100 to about 300 angstroms.

In another aspect, the invention provides a semiconductor circuitcomprising a semiconductor structure comprising a resistance variablematerial interposed between and in electrical contact with first andsecond electrodes. The resistance variable material comprises achalcogenide material having metal ions dispersed therethrough. In oneembodiment of the semiconductor circuit, a diffusion layer comprising asputter deposited (or CVD deposited) conductive metal is disposed overthe resistance variable material, and a plated layer of a conductivemetal overlies the diffusion layer to form the second (upper) electrode.The conductive metal forming the second (upper) electrode can also beplated onto a thin seed layer of a conductive metal that is depositedonto the diffusion layer and/or the resistance variable material. Theresistance variable material can be disposed within an opening throughan insulative layer that overlies the first electrode.

In another embodiment of a semiconductor circuit, the semiconductorstructure comprises a resistance variable material comprising ametal-chalcogenide material, sandwiched between an upper and lowerelectrode. A thin diffusion layer of an electrolessly plated conductivemetal is disposed over an activated surface area overlying theresistance variable material, and a plated conductive metal layeroverlies the diffusion layer to form the upper electrode. The activatedsurface area comprises a surface activating agent, for example,palladium. If appropriate, the plated conductive metal layer forming theupper electrode can overlie a plated seed layer of a conductive metaldisposed over the diffusion layer and/or the resistance variable layer.

In yet another aspect, an integrated circuit is provided. In oneembodiment, the integrated circuit comprises a substrate; a firstelectrode comprising a conductive material formed over the substrate; aninsulating dielectric layer (if appropriate) formed over the firstelectrode with an opening formed into the insulating layer to expose thefirst electrode; a layer of a resistance variable material comprising ametal-chalcogenide material (formed in the opening) in electricalcontact with the first electrode; a sputtered or CVD deposited diffusionlayer overlying the resistance variable material; and a second electrodeplated onto the diffusion layer or a thin seed layer (whereappropriate). Where appropriate, an insulating dielectric layer isformed over the substrate and the first electrode is formed over theinsulating layer. In a preferred embodiment, the chalcogenide materialcomprises germanium selenide, the diffusion layer comprises silver andis preferably about 100 to about 300 angstroms thick, and the platedelectrode layer comprises a solid silver layer.

In another embodiment of an integrated circuit according to theinvention, the circuit comprises a substrate; a first electrodecomprising a conductive material formed over the substrate; aninsulating dielectric layer (if appropriate) formed over the firstelectrode with an opening formed into the insulating layer to expose thefirst electrode; a layer of a resistance variable material comprising ametal-chalcogenide material (formed in the opening) in electricalcontact with the first electrode; an activated surface area formed overthe resistance variable material and comprising a surface activatingagent; an electrolessly plated diffusion layer formed over the activatedsurface area of the resistance variable material; and a second platedconductive metal formed over the diffusion layer to form an upperelectrode. The conductive metal of the second electrode can be platedonto a metal seed layer. In a preferred embodiment, the chalcogenidematerial comprises germanium selenide, the surface activating agentcomprises palladium, the diffusion layer comprises electrolessly platedsilver, and the second electrode comprises a plated silver layer.

Conventional methods of diffusing metal into the chalcogenide materialto form a resistance variable material layer, and then sputterdepositing the upper electrode over the layer, results in excess metalions being diffused into the chalcogenide material and a failed device.Advantageously, the present methods of sputtering (or CVD depositing) avery thin metal (e.g., silver) diffusion layer onto the surface of thechalcogenide layer or electrolessly plating a diffusion layer onto theactivated surface of the chalcogenide layer, and then plating additionalmetal over the diffusion layer to form the upper electrode is useful incontrolling and reducing the diffusion of metal ions into thechalcogenide material and in forming memory devices comprising aresistance variable element that are operable and functional.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings, which are forillustrative purposes only. Throughout the following views, thereference numerals will be used in the drawings, and the same referencenumerals will be used throughout the several views and in thedescription to indicate same or like parts.

FIG. 1A is a diagrammatic cross-sectional view of a semiconductor waferfragment at a preliminary step of a processing sequence.

FIGS. 1B through 1E are views of the wafer fragment of FIG. 1A atsubsequent and sequential processing steps, showing fabrication of anelectrode according to an embodiment of the method of the invention.

FIG. 2A is a diagrammatic cross-sectional view of a semiconductor waferfragment at a preliminary step of a processing sequence.

FIGS. 2B through 2F are views of the wafer fragment of FIG. 2A atsubsequent and sequential processing steps, showing fabrication of aconductive contact according to another embodiment of the method of theinvention.

FIGS. 3A-3C are cross-sectionals view of scanning electromicrographs(SEMs) of a series of wafers comprising a germanium selenide layer, anoverlying silver diffusion layer and a layer of silver electroplatedonto the diffusion layer as described in Example 1. FIGS. 3A′-3C′ arecross-sectionals view of SEMs of the corresponding Controls for FIG. 3C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described generally with reference to the drawingsfor the purpose of illustrating the present preferred embodiments onlyand not for purposes of limiting the same. The figures illustrateprocessing steps for use in the fabrication of semiconductor devices inaccordance with the present invention. It should be readily apparentthat the processing steps are only a portion of the entire fabricationprocess.

In the current application, the terms “semiconductive wafer fragment” or“wafer fragment” or “wafer” will be understood to mean any constructioncomprising semiconductor material, including but not limited to bulksemiconductive materials such as a semiconductor wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structureincluding, but not limited to, the semiconductive wafer fragments orwafers described above. The terms “fast ion conductor” and/or“resistance variable material” refer to a metal ion-containing glass, ametal ion-containing amorphous semiconductor, a chalcogenide-metal ion,a superionic conductor, and the like.

FIGS. 1A through 1E depict a method of forming a non-volatile resistancevariable memory device according to a first embodiment of a method ofthe invention.

Referring to FIG. 1A, a wafer fragment 10 is shown at a preliminaryprocessing step in the formation of a resistance variable memory device.The wafer fragment 10 in progress can comprise a semiconductor wafersubstrate or the wafer along with various process layers formed thereon,including one or more semiconductor layers or other formations, andactive or operable portions of semiconductor devices.

The wafer fragment 10 comprises a substrate 12 such as plastic, glass orsemiconductor material, to provide support for the device. Ifappropriate, an overlying insulating layer 14 such as silicon dioxide,is provided to insulate the substrate 12 from the active portion of thedevice. A lower electrode 16 is formed over the insulating layer 14 (orover the substrate 12 if no insulator is used). The lower electrode canbe formed on the insulating layer or substrate by methods known and usedin the art. For example, the lower electrode can be formed by depositinga thin metal seed layer or by activating the surface using an activatingagent such as palladium (PdCl₂) and then depositing a conductive metalto form the lower electrode by electro- or electroless plating. Thelower electrode 16 can constitute a patterned line over the insulatinglayer, and preferably comprises silver or other conducting material suchas nickel, chromium, cobalt, molybdenum, tungsten, palladium, platinum,aluminum, titanium, copper, and carbon, or combinations and mixturesthereof, being tungsten in the illustrated example.

In an exemplary embodiment, a chalcogenide material 18 is deposited ontothe lower electrode 16, as depicted in FIG. 1A. Chalcogenide materialsare known in the art and include, but are not limited to, compoundshaving the formula B_(x)A_(y) where “B” is arsenic (As) or germanium(Ge), and where “A” is selenium (Se), tellurium (Te), sulfur (S), or amixture thereof. A preferred chalcogenide material 18 is represented bythe formula Ge_(x)A_(y) and includes, for example, germanium sulfide andgermanium selenide. Another example of a chalcogenide material isrepresented by the formula As_(x)A_(y), and includes, for example,arsenic sulfide and arsenic trisulfide (As₂S₃). The chalcogenidematerial 18 can be formed over the lower electrode 16 by methods knownand used in the art, for example, by chemical vapor deposition, physicalvapor deposition, or evaporation.

According to the presently described embodiment of a method of theinvention, a thin diffusion layer 20 is initially deposited onto thechalcogenide material 18, as depicted in FIG. 1B. The diffusion layer 20can be deposited by chemical vapor deposition (CVD), and is preferablydeposited by a conventional physical vapor deposition (i.e., sputtering)process. The diffusion layer 20 can comprise a conducting material, forexample, silver, copper or zinc, being silver in the illustratedexample.

As discussed above, sputter deposition of metal such as silver onto agermanium selenide or other chalcogenide material can result in excessdiffusion of metal ions into the chalcogenide material due to UVradiation generated during prolonged sputtering, resulting in a faileddevice component. Therefore, it is desirable to limit the thickness ofthe sputtered diffusion layer 20 to lessen the amount of UV radiationthat is generated, and minimize the diffusion of silver or other metalinto the underlying chalcogenide material 18. The thickness of thediffusion layer 20 is preferably at least about 100 angstroms to about300 angstroms.

In the illustrated embodiment, the deposited silver diffusion layer 20can then be treated by exposure to electromagnetic radiation or otherdiffusion source 22 to diffuse silver ions from the diffusion layer 20into the underlying chalcogenide layer 18, resulting in themetal-comprising resistance variable material or fast ion conductorlayer 24, as depicted in FIG. 1C. The substrate can also be heatedduring an irradiation process, for example, to a temperature of about65° C. to about 100° C. Since the present method forms the upperelectrode 28 by limiting sputtering of silver or other metal to formingthe thin diffusion layer 20 and plating the mass of the upper electrode,little electromagnetic radiation is produced during the formation of theupper electrode such that only a minimal amount of metal ions is driveninto the resistance variable material layer 24 during the formation ofthe upper electrode.

A conformal metal layer 26 is then plated over the diffusion layer 20,as depicted in FIG. 1D, to form the upper electrode 28. A conventionalelectroplating process or an electroless plating process can be used.Where the diffusion layer 20 has been consumed during the diffusing stepof the metal ions and is less than about 100 angstroms, it is desirableto deposit a thin seed layer (not shown) onto the resistance variablematerial layer 24 (or remainder of the diffusion layer) to form a basefor the subsequent deposition of the upper electrode. The seed layer canbe deposited by CVD or PVD, but is preferably a plated layer. The upperelectrode 28 preferably comprises silver or other conductive materialsuch as copper, zinc, or platinum, among others, being silver in theillustrated example. The diffusion layer 20 (or seed layer) and theplated electrode layer 28 can comprise the same metal, or differentmetals if adherent to each other. Preferably, the diffusion layer 20 (orseed layer) and the plated electrode layer 26 comprise the same metal toachieve a high level of adhesion and conductivity between the twolayers. Formation of the conformal electrode layer 26 progressesuniformly from the diffusion layer 20 (or seed layer if used) by theattachment or fusing of metal from the plating solution to the diffusionlayer 20 (or seed layer).

In an example of a suitable silver electroplating process, the wafer 10is immersed in a bath comprising an aqueous solution of a silver saltsuch as silver nitrate (AgNO₃), potassium silver cyanide (KAg(CN)₂) orsilver succinimide (C₄H₅O₂NAg). Such silver plating baths arecommercially available and used in the art. One preferred solution is acyanide-free silver plating solution comprising silver succinimide(C₄H₅O₂NAg), which is available commercially under the trade nameTechni-Silver CY-LESS® L2 from Technic, Inc., Cranston, R.I.

A current is applied to reduce the metal ions and deposit a silver metalelectrode layer 26 onto the silver diffusion layer 20. The substrateremains immersed in the solution bath and the current is applied untilthe desired thickness of silver is obtained, preferably a thickness ofabout to 400 to about 1500 angstroms, or a total thickness (layers 20and 26) of about 500 to about 2000 angstroms. Preferably, the coatingsolution is gently agitated to more uniformly plate the electrode on thesurface of the resistance variable material layer 24.

The current is terminated when the desired thickness is reached, and thesubstrate is placed into a water rinse bath to remove residual bathliquid and particles from the surface of the wafer and then dried. Thesubstrate is then immersed in an organic solvent such as acetone or analcohol such as ethanol or isopropanol that is miscible with the rinseliquid and with water, and then spin dried.

In another embodiment of a plating system, an electroless platingprocess can also be used to deposit a conformal metal electrode layer 26(e.g., silver, copper) to form the upper electrode 28. In an exemplaryelectroless plating process for forming a silver electrode, thesubstrate is immersed in a bath comprising an aqueous solution of asilver salt (e.g., AgNO₃, KAg(CN)₂), a chemical reducing agent, andoptional additives as known and used in the art to control stability,film properties, deposition rates, and to build up the metal coating.The metal ions are reduced by reaction with the chemical reducing agentin the plating solution and, as depicted in FIG. 1D, deposit as a silverelectrode layer 26 onto the silver diffusion layer 20 to a desiredthickness.

A preferred chemical reducing agent for use with a silver electrolessplating process comprises a mixture of glucose and tartaric acid in aratio (g/g) of about 1:0.1. Examples of other suitable chemical reducingagents include organic and inorganic compounds such as potassiumhypophosphite, ammonium hypophosphite, aldehydes such as formaldehydeand glyoxal, potassium borohydride, amine boranes, hydrazine, hydrazinesulfate, Rochelle salt (potassium sodium tartrate), ascorbic acid, andthe like.

Optional additives can be added to increase the rate of deposition,increase the stability of the bath, and/or function as a buffer or mildcomplexing agent. Examples of suitable additives include acetic acid,sodium acetate, sodium fluoride, lactic acid, propionic acid, sodiumpyrophosphate, ethylenediamine, thallous nitrate, boric acid, citricacid, hydrochloric acid, malonic acid, glycine, malic acid,mercaptobenzothiazole, sodium lauryl sulfate, lead (II) ion, sodiumpotassium tartrate, ammonium hydroxide, potassium hydroxide, sodiumhydroxide, sodium carbonate, ethylendiaminetetraacetic acid,mercaptobenzothiazole, methyldichlorosilane and tetrasodiumethylenediaminetetraacetic acid, sodium citrate, ammonium chloride,ammonium sulfate, sodium lauryl sulfate, sodium succinate, sodiumsulfate and the like, and mixtures thereof.

In a silver electroless plating process, the thickness of the upperelectrode 28 is controlled by the length of time that the substrate isimmersed in the aqueous coating solution. Preferably, the substrate isimmersed in the bath to deposit a quantity of silver onto the seed layer20 to a thickness of about 400 to about 1500 angstroms, or a totalthickness (diffusion layer 20+electrode layer 26) of about 500 to about2000 angstroms. The solution can be gently agitated to more uniformlyand efficiently deposit silver onto the diffusion layer 20 to form theelectrode layer 26.

To terminate the metal deposition in an electroless plating, thesubstrate is removed and placed in a rinse bath. The rinse bath cancomprise water, an alcohol such as ethanol or isopropanol, or otherorganic liquid that is chemically inert to the silver coating and willdisplace the coating solution from the substrate surface. The substrateis held in the rinse bath until the coating solution has been displaced.Preferably, the rinse solution is gently agitated to more rapidly removethe coating solution from the surface. After rinsing, the substrate canbe removed from the rinse bath and allowed to dry, or can be spin dried.

The electrode layer 26 (and diffusion layer 20) can then be patterned toform the upper electrode 28, as depicted in FIG. 1E, by conventionalphotolithography processing using a photoresist mask and a wet or,preferably, a dry etch procedure. The electrode layer 26 can also bepartially removed by conventional chemical mechanical polishing (CMP).

In use, when a voltage is applied, a dendrite (not shown) is formedbetween the upper and lower electrodes.

According to this embodiment of the method of the invention, thediffusion of metal ions into the chalcogenide material 18 to form theresistance variable material layer 24 is controlled by limiting thesilver (or other metal) sputtering step to the formation of a thindiffusion layer, and then depositing the mass of the electrode byplating. The present process controls the diffusion of metal ions intothe chalcogenide material to achieve a suitably functional memorydevice.

Referring now to FIGS. 2A-2F, in another embodiment of a methodaccording to the invention, the surface 36′ of the resistance variablematerial layer 18′ is pre-activated and a metal, for example, silver orcopper, being silver in the illustrated example, is deposited onto theactivated areas by electroless plating to form a diffusion layer 25′. Aconformal layer 26′ of the same or a different metal can be deposited byelectroless or electroplating to form the upper electrode layer 28′,also being silver in the illustrated example.

Referring to FIG. 2A, a wafer fragment 10′ is shown before processing.Briefly, wafer fragment 10′ includes a supportive substrate 12′, anoverlying insulative layer 14′ (if appropriate), a lower electrode layer16′, and an overlying chalcogenide material 18′, which is preferablygermanium selenide. As depicted, an insulating layer 14′ has been formedover the substrate 12′, and the chalcogenide material 18′ has beendeposited within an opening 32′ (e.g., via) formed through theinsulating layer 34′ and overlying the lower electrode 16′.

According to the invention, the surface 36′ of the chalcogenide material18′ is contacted with a surface-activating agent to form an activatedsurface area(s) 38′ over layer 18′, as depicted in FIG. 2B. Thesurface-activating agent functions to chemically activate the surface36′ of the chalcogenide material 18′ for initiating metal deposition inan electroless plating process. A preferred surface-activating agentcomprises a colloidal suspension of palladium (Pd) or palladium chloride(PdCl₂) as a catalyst, and a tin (Sn) species such as stannous chloride(SnCl₂) as a stabilizer, as known and used in the art. The wafer can bedipped into an aqueous solution comprising the surface-activating agentto coat the chalcogenide material 18′, preferably for about 10 to about150 seconds. A multi-step process can also be used, for example, bydipping the wafer in a SnCl₂ solution, rinsing the wafer, and thendipping the wafer in a palladium or PdCl₂ solution.

The wafer is then placed into a rinse bath comprising water or othercompatible solvent, for example, an alcohol such as ethanol orisopropanol, to remove the surface-activating agent solution. Activatedsurface areas 38′ on the chalcogenide material 18′ comprise small islandnucleation sites of palladium which initiate deposition of silverthereon in a subsequent silver plating step.

Referring to FIG. 2C, silver is plated as a conformal diffusion layer25′ onto the activated surface areas 38′ of the chalcogenide material18′ in an electroless plating process by immersing the substrate in anaqueous bath comprising silver metal ions and a chemical reducing agent,as discussed above. The plating step proceeds until the diffusion layer25′ is formed, preferably to a thickness of about 300 angstroms or less,preferably about 100 to about 300 angstroms. The substrate is thenremoved from the metal ion bath, placed in a rinse bath to terminatedeposition, and dried.

The diffusion layer 25′ is then exposed to electromagnetic radiation orother diffusion source 22′ to diffuse metal ions (e.g., silver ions)from the diffusion layer 25′ into the chalcogenide layer 18′, resultingin the resistance variable material layer 24′, as depicted in FIG. 2D.

Referring to FIG. 2E, a conformal metal layer 26′ is then plated overthe diffusion layer 25′ in an electroless or electroplating process. Theplating step proceeds until a desired mass and thickness is achieved,preferably to a thickness of about 500 to about 2000 angstroms. Wherethe diffusion layer 25′ has been depleted from the foregoing diffusionstep, a seed layer (not shown) of a conducting metal can be plated ontothe resistance variable material layer 24′ as a base for the conformalmetal layer 26′ of the top electrode. Where the underlying diffusionlayer 25′ is less than about 100 angstroms, the seed layer is preferablyplated using an electroless plating process.

As depicted in FIG. 2F, the conformal metal layer 26′ (and diffusionlayer 25′) can be patterned by conventional photolithographic methodsand/or partially removed by conventional CMP to form the upper electrode28′.

In the present embodiment of the method, diffusion of metal ions intothe chalcogenide material to form the resistance variable material layeris controlled by omitting the metal sputtering step and forming thediffusion layer 25′ by electroless plating, diffusing metal into thechalcogenide layer 18′ from the diffusion layer 25′, and then depositingthe upper electrode layer 26′ by an electroless or electroplatingprocess.

The resulting non-volatile resistance variable devices can be used in avariety of applications including, for example, programmable memorydevices, programmable resistor and capacitor devices, optical devices,sensors, among others.

EXAMPLES 1-3 Electroplating of Silver Electrode onto a Silver Seed Layer

A silver electrode was formed over a layer of germanium selenide on asemiconductor wafer according to an embodiment of the method of theinvention.

In a series of experimental wafers, the electrode was formed by firstsputtering a layer of silver onto the germanium selenide layer to form aseed layer, and then plating silver onto the seed layer for a designatedplating time by conventional electroplating using a EG&G model 270 andthe processing parameters shown. The thickness of the seed layer variedfrom 250 angstroms to 400 angstroms.

With the Control wafers, the electrode was formed by sputtering a layerof silver onto the germanium selenide layer to a thickness of 150 to 700angstroms.

EXAMPLE 1

-   Solution: 100 ml of Techni-Silver CY-LESS® L2 (Technic, Inc.,    Cranston, R.I.)-   Wafer: ca. 4 cm×5 cm; Ag seed layer thickness=250 Å-   Potential: −0.05 V-   Plating Time: 120 seconds-   Temperature: ambient-   Resistance:    -   Pre-Plating (avg.): 2.76 ohms/square    -   Post-Plating (avg.): 0.66 ohms/square-   SEMs: FIG. 3A (Experimental)

EXAMPLE 2

-   Solution: 100 ml of Techni-Silver CY-LESS® L2 (Technic, Inc.,    Cranston, R.I.)-   Wafer: ca. 4 cm×5 cm; Ag seed layer thickness=150 Å-   Potential: −0.05 V-   Plating Time: 120 seconds-   Temperature: ambient-   Resistance:    -   Pre-Plating (avg.) 5.7 ohms/square    -   Post-Plating (avg.): 1.70 ohms/square-   SEMs: FIG. 3B (Experimental)

EXAMPLE 3

-   Solution: 100 ml of Techni-Silver CY-LESS® L2 (Technic, Inc.,    Cranston, R.I.)-   Wafer: ca. 4 cm×5 cm; Ag seed layer thickness=ca. 600 Å-   Potential: −0.05 V-   Plating Time: 120 sec-   Temperature: ambient-   Resistance:    -   Pre-Plating (avg.): 1.44 ohms/square    -   Post-Plating (avg.): 0.96 ohms/square-   SEMs: FIG. 3C (Experimental)    -   FIGS. 3A′, 3B′, 3C (Control)

A decrease in resistance is evidence of increased silver thickness,proving that plating was successful. This is demonstrated further by the(SEM) images shown in FIGS. 3A-3C. The results demonstrated that a thinsputtered silver layer can be used as a plating seed to further increasethe silver thickness without the generation of electromagneticradiation.

EXAMPLES 4 and 5 Electroless Plating of Silver Electrode onto a SilverSeed Layer

A silver electrode was formed over a layer of germanium selenide on asemiconductor wafer, according to a method of the invention.

In a series of experimental wafers, a silver electrode was formed byfirst sputtering a layer of silver onto a germanium selenide layer on asemiconductor wafer, to form a seed layer. The thickness of the seedlayer varied from 150 angstroms to 400 angstroms. Silver was then platedonto the seed layer for a designated plating time by conventionalelectroless plating using the processing parameters shown below.

With the Control wafers, the electrode was formed by sputtering a layerof silver onto the germanium selenide layer on a wafer.

EXAMPLE 4

-   Plating Solution:    -   AgNO₃ (0.3 g)    -   NH₄OH (0.3 ml)    -   KOH (0.5 ml)    -   Glucose (3.87 g)    -   Tartaric acid (0.35 g)    -   Isopropyl alcohol (IPA) (8.75 ml)    -   H₂O (total of 92.7 ml)-   Wafer: ca. 4 cm×4 cm, Ag seed layer thickness: 200 angstroms.-   Plating Time: 300 seconds-   Temperature: Ambient to begin with, but plating was very slow. After    15 minutes, the bath was slightly warmed. The bath decomposed upon    heating within 15 minutes.-   Resistance    -   Pre-Plating (avg.): 4.22 ohms/square    -   Post-Plating (avg.): 2.52 ohms/square

EXAMPLE 5

-   Plating Solution:    -   AgNO₃ (0.35 g)    -   NH₄OH (until precipitate dissolved)    -   KOH (0.35 ml)    -   Glucose (4.50 g)    -   Tartaric acid (0.4 g)    -   Ethanol (10 ml)    -   H₂O (total of 106 ml)-   Wafer: ca. 4 cm×4 cm, Ag seed layer thickness: 200 angstroms.-   Plating Time: 300 seconds-   Temperature: Ambient to begin with, but plating did not seem to    start of was very slow. After 15 minutes, the bath was slightly    warmed to about 30° C. The bath decomposed upon heating within 30    minutes.-   Resistance    -   Pre-Plating (avg.): 4.51 ohms/square    -   Post-Plating (avg.): 2.37 ohms/square

The silver layer formed on the experimental wafers by electrolessplating onto the silver seed layer over the germanium selenide layer hadan increased thickness as evidenced by the decrease in resistance afterplating.

The experimental silver layers also had a greater thickness compared tothe corresponding Control wafers. The resistance of the silver layer inthe Control wafers was about 4.5 ohms/square (pre-plating), and about2.3 ohms/square (post-plating). This indicates that the silver thicknessincreased from about 200 to 300 angstroms.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of forming a memory device, comprising the steps of:providing a substrate comprising a first electrode and an overlyingchalcogenide material layer; chemical vapor depositing a diffusion layercomprising a conductive metal onto the chalcogenide material layer;diffusing metal ions from the diffusion layer into the chalcogenidematerial layer to form a resistance variable material layer; and platinga conductive metal onto the resistance variable material layer oroverlying remaining diffusion layer to form a second electrode such thatthe metal ions within the resistance variable material layer are atabout the same level as before the plating step.
 2. The method of claim1, wherein the diffusion layer comprises a conductive metal selectedfrom the group consisting of silver, copper, and zinc.
 3. The method ofclaim 1, wherein the diffusion layer comprises silver.
 4. The method ofclaim 1, wherein the step of plating comprises an electroplatingprocess.
 5. The method of claim 1, wherein the step of plating comprisesan electroless plating process.
 6. The method of claim 1, furthercomprising, prior to the plating step, the step of depositing aconductive metal seed layer over the resistance variable material layeror overlying remaining diffusion layer.
 7. The method of claim 1,wherein the second electrode comprises a conductive metal selected fromthe group consisting of silver, copper, zinc, and platinum.
 8. Themethod of claim 1, wherein the second electrode comprises silver.
 9. Themethod of claim 1, wherein the second electrode comprises copper. 10.The method of claim 1, wherein the second electrode and the diffusionlayer comprise the same conductive metal.
 11. The method of claim 10,wherein the diffusion layer and the second electrode comprise silver.12. The method of claim 1, wherein the diffusion layer and the secondelectrode layer comprise different metals.
 13. The method of claim 12,wherein the diffusion layer comprises silver, and the second electrodecomprises a conductive metal other than silver.
 14. The method of claim1, wherein the step of diffusing the metal ions into the chalcogenidematerial comprises exposing the diffusion layer to electromagneticradiation.
 15. A method of forming a memory device, comprising the stepsof: providing a substrate comprising a layer of conductive material andan overlying layer of a chalcogenide material; physical vapor depositinga diffusion layer comprising a conductive metal onto the chalcogenidematerial layer; diffusing metal ions from the diffusion layer into thechalcogenide material layer to form a resistance variable material layercomprising an effective level of the metal ions; and plating a layer ofa conductive metal onto the resistance variable material layer oroverlying remaining diffusion layer such that the level of the metalions within the resistance variable material layer remains about thesame as before the plating step.
 16. A method of forming a programmablememory device, comprising the steps of: providing a substrate comprisinga chalcogenide material overlying and in electrical contact with a firstelectrode; forming a diffusion layer by chemical vapor depositing afirst conductive metal onto the chalcogenide material; forming thechalcogenide material into a resistance variable material by diffusingmetal ions from the diffusion layer into the chalcogenide material; andforming a second electrode by plating a second conductive metal onto theresistance variable material layer or overlying remaining diffusionlayer such that the metal ions within the resistance variable materialare at about the same level as before forming the second electrode. 17.The method of claim 16, wherein the step of forming the second electrodecomprises electroplating the second conductive metal.
 18. The method ofclaim 16, wherein the step of forming the second electrode compriseselectroless plating the second conductive metal.
 19. A method of forminga programmable memory device, comprising the steps of: providing asubstrate comprising a chalcogenide material overlying and in electricalcontact with a first electrode; forming a diffusion layer by physicalvapor depositing a first conductive metal onto the chalcogenidematerial; forming the chalcogenide material into a resistance variablematerial by diffusing metal ions from the diffusion layer into thechalcogenide material; and forming a second electrode by plating asecond conductive metal onto the resistance variable material layer oroverlying remaining diffusion layer such that the metal ions within theresistance variable material are at about the same level as beforeforming the second electrode.
 20. The method of claim 19, wherein thestep of forming the second electrode comprises electroplating the secondconductive metal.
 21. The method of claim 19, wherein the step offorming the second electrode comprises electroless plating the secondconductive metal.
 22. A method of forming a non-volatile resistancevariable device, comprising the steps of: forming a first electrode on asubstrate; forming a dielectric layer over the first electrode; formingan opening through the dielectric layer to the first electrode;depositing a layer of a chalcogenide material into the opening incontact with the first electrode; chemical vapor depositing a conductivemetal onto the chalcogenide material layer to form a diffusion layer;diffusing metal ions from the diffusion layer into the chalcogenidematerial layer to form a resistance variable material layer; and platinga layer of a conductive metal onto the resistance variable materiallayer or overlying remaining diffusion layer to form a second electrodesuch that the metal ions within the resistance variable material layerare at about the same level as before the plating step.
 23. The methodof claim 22, comprising forming the non-volatile resistance variabledevice into a programmable memory cell of a memory circuitry.
 24. Amethod of forming a non-volatile resistance variable device, comprisingthe steps of: forming a first electrode on a substrate; forming adielectric layer over the first electrode; forming an opening throughthe dielectric layer to the first electrode; depositing a layer of achalcogenide material into the opening in contact with the firstelectrode; physical vapor depositing a conductive metal onto thechalcogenide material layer to form a diffusion layer; diffusing metalions from the diffusion layer into the chalcogenide material layer toform a resistance variable material layer; and plating a layer of aconductive metal onto the resistance variable material layer oroverlying remaining diffusion layer to form a second electrode such thatthe metal ions within the resistance variable material layer are atabout the same level as before the plating step.
 25. A method ofcontrolling diffusion of metal into a chalcogenide material to form aresistance variable material, comprising the steps of: providing asubstrate comprising a chalcogenide material; chemical vapor depositinga diffusion layer comprising a conductive metal onto the chalcogenidematerial to a thickness of about 100-300 angstroms; diffusing metal ionsinto the chalcogenide material to form the resistance variable material;and plating a layer of a conductive metal over the resistance variablematerial layer or overlying remaining diffusion layer to a desiredthickness such that the metal ions within the resistance variablematerial are at about the same level as before the plating step.
 26. Amethod of controlling diffusion of metal into a chalcogenide material toform a resistance variable material, comprising the steps of: providinga substrate comprising a chalcogenide material; physical vapordepositing a diffusion layer comprising a conductive metal onto thechalcogenide material to a thickness of about 100-300 angstroms;diffusing metal ions into the chalcogenide material to form theresistance variable material; and plating a layer of a conductive metalover the resistance variable material layer or overlying remainingdiffusion layer to a desired thickness such that the metal ions withinthe resistance variable material are at about the same level as beforethe plating step.
 27. A memory device, prepared by the process of:forming a first electrode on a substrate; depositing a layer of achalcogenide material over the first electrode; chemical vapordepositing a conductive metal onto the chalcogenide material layer toform a diffusion layer; diffusing metal into the chalcogenide materiallayer to form a resistance variable material layer; and plating aconductive metal over the resistance variable material layer oroverlying remaining diffusion layer to form a second electrode such thatthe metal ions within the resistance variable material layer are atabout the same level as before the plating step.
 28. A memory device,prepared by the process of: forming a first electrode on a substrate;depositing a layer of a chalcogenide material over the first electrode;physical vapor depositing a conductive metal onto the chalcogenidematerial layer to form a diffusion layer; diffusing metal into thechalcogenide material layer to form a resistance variable materiallayer; and plating a conductive metal over the resistance variablematerial layer or overlying remaining diffusion layer to form a secondelectrode such that the metal ions within the resistance variablematerial layer are at about the same level as before the plating step.